feat: Refactor playlist management, error handling, and testing infrastructure

- Improved playlist item handling with pagination, error logging, and dedicated request hooks.
- Consolidated reusable helpers for track ID normalization, playlist validation, and removal logic.
- Enhanced error handling by introducing `PlaylistNotFound` and `UserNotAuthenticated` exceptions.
- Refined Pytest `qapp` fixture for headless CI compatibility and session-scoped reuse.
- Replaced redundant fixtures in multiple test files for consistency.
- Improved playlist management GUI with detailed debug logging for notifications and operations.
- Added `deptry` configuration to handle transitive dependencies and ignored exclusions.
- Updated Pytest and Tox configurations for consistent environment setup during CI/CD runs.

tox.ini

@@ -10,6 +10,9 @@ python =
 [testenv]
 passenv = PYTHON_VERSION
 allowlist_externals = poetry, pytest
+setenv =
+    QT_QPA_PLATFORM=offscreen
+    XDG_RUNTIME_DIR={toxinidir}/.xdg
 commands =
     poetry install -v --no-interaction --all-extras --with dev,docs
     pytest --doctest-modules tests --cov --cov-config=pyproject.toml --cov-report=xml